| Abstrakt: |
Changes in the levels of plasma vitellogenin (Vg), estradiol (E2) and testosterone (T) were examined following gonadal development induced by carp gonadotropin treatment (cGTH) of freshwater female yellow and silver eels (Anguilla anguillaL.). The animals received injections of cGTH (250 µg kg−1body weight) or saline vehicle three times a week, for 6 to 8 weeks. No effect of vehicle was observed. Steroidogenic activity of the ovary was stimulated by cGTH treatment as shown by the increase in circulating steroid levels in both stages. However, the responses of T, E2and Vg differed according to the stage of development of eels. At the yellow stage, the initial steroid plasma levels were undetectable (< 0.01 ng ml−1) before treatment and ovarian steroidogenic activity was slightly stimulated following cGTH treatment; steroid levels reached their highest values after 3 weeks and 6 weeks of treatment for E2(0.62 ± 0.13 ng ml−1and T (0.33 ± 0.30 ng ml−1), respectively. The cGTH treatment slightly increased plasma Vg levels (0.2–0.7 µg ml−1during the experiment compared with the initial values of the group. At the silver stage, the initial steroid levels were detectable (0.7 ng ml−1for E2and 0.1 ng ml−1for T); cGTH treatment did not significantly increase plasma E2level which remained at initial levels. Nevertheless, plasma T levels dramatically increased from 0.1 to 3 ng ml−1and peaked after 1 or 2 weeks of cGTH treatment; a rapid increase in plasma Vg levels occurred, reaching its highest value at 5 mg ml−1after 3 weeks of treatment. Thus, the steroid kinetic profiles in relation to the appearance of Vg in the plasma following cGTH treatment was closely related to androgen levels and there was a strong vitellogenic response induced by chronic cGTH treatment. In order to test if androgens could be implicated in the vitellogenic response, we evaluated the potencies of various androgens (testosterone and 5α-androstane-3β,17β-diol)in vivoandin vitro, associated with E2to induce the production of Vg.In vitroexperiments showed that Vg synthesis was induced by high doses (10−6to 10−5M) of androgen in the eel. Tamoxifen totally inhibited the action of androgens suggesting that androgens were acting through binding to the E2receptor.In vivo, androgens given alone at 50 µg kg−13 times a week for 1 months had no significant effect on plasma Vg levels. In addition, E2-androgen cotreatment showed that the presence of androgen did not modify the vitellogenic response induced by E2. |
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